1. Field of the Invention
This invention relates to a multi-plate solid-state color imaging apparatus with separate solid-state imaging elements for the individual color components, and more particularly to improvements in a multi-plate solid-state color imaging apparatus using what is called a spatial pixel-shifting method, in which the solid-state imaging elements are shifted from each other horizontally so that a light-insensitive portion of each pixel of the solid-state imaging element located in a reference position may align with a light-sensitive portion of each pixel of the other solid-state imaging elements.
2. Description of the Related Art
For known multi-plate solid-state color imaging apparatuses, three-plate solid-state color imaging apparatuses using three solid-state imaging elements have recently been popularized. FIG. 1 shows a general construction of such a three-plate solid-state color imaging apparatus. In FIG. 1, numeral 11 indicates a camera lens. The optical image of a subject passing through the camera lens 11 is resolved by a color separation prism 12 into three color components, an R (red) component, a G (green) component, and a B (blue) component, which are then focused on three solid-state imaging elements 13, 14, and 15 provided for the respective color components, and are converted into electric signals.
The photoelectric conversion of the three solid-state imaging elements 13, 14, and 15 is controlled by the driving pulses generated by a driving circuit 17 based on the pulse signal supplied from a pulse generator circuit 16.
The signals supplied from the solid-state imaging elements 13, 14, and 15 are amplified to a specified level by amplifier circuits 18, 19, and 20, respectively, and then undergo white balance adjustment. The resulting signals are supplied to process circuits 21, 22, and 23, respectively, and then undergo nonlinear processing including gamma correction.
The respective output signals E.sub.R, E.sub.G, and E.sub.B of the process circuits 21, 22, and 23 are supplied to an encoder circuit 24, which encodes these signals into a television signal of, for example, the NTSC system, one of standard television systems. This produces a television signal corresponding to the optical image of the subject. The television signal thus produced is taken out from the output terminal 25 and used, for example, to display the image on a television set or record the image on a recording medium such as a magnetic tape.
Solid-state imaging elements have a poorer resolution than conventional high-performance camera tubes. Thus, for the above-described three-plate solid-state color imaging apparatus, the horizontal spatial arrangement of the three solid-state imaging elements 13, 14, and 15 is improved for better horizontal resolution.
The improvement is such that, for example, the mounting position of the G-component solid-state imaging element 14 is used as a reference, and the G-component solid-state imaging element 14, and the remaining R-component and B-component solid-state imaging elements 13 and 15 are placed in such a manner that they are shifted half a pixel pitch from each other horizontally so that a light-insensitive portion of each pixel of the solid-state imaging element 14 in the reference position may align with a light-sensitive portion of each pixel of the other R-component and B-component solid-state imaging elements 13 and 15. This is what is called a spatial pixel-shifting method.
With the spatial pixel-shifting method, if the phase of the electric signal from, for example, the solid-state imaging element 14 is used as the reference phase or zero phase, and the phase of the electric signals from the solid-state imaging elements 13 and 15 is set as .pi. phase 180.degree. shifted from the zero phase, when the encoder circuit 24 produces a luminance signal from the respective output signals E.sub.R, E.sub.G, and E.sub.B of the process circuits 21, 22, and 23, synthesizing the outputs E.sub.R, E.sub.G, and E.sub.B at a mixing ratio of 0.30 E.sub.R +0.59 E.sub.G +0.11 E.sub.B seems to roughly double the number of pixels in the horizontal direction, providing a higher resolution.
Specifically, in a case where an achromatic subject is shot under standard illumination, when the white balance is suitably adjusted, the level ratio of the respective output signals V.sub.R, V.sub.G, and V.sub.B of the amplifier circuits 18, 19, and 20 will be: EQU V.sub.R :V.sub.G :V.sub.B =1:1:1 (1)
As a result of this, the level ratio of the respective output signals E.sub.R, E.sub.G, and E.sub.B of the process circuits 21, 22, and 23 will be: EQU E.sub.R :E.sub.G :E.sub.B =1: 1:1 (2)
Since the luminance signal synthesized at the encoder circuit 24 is made up of zero-phase signal E.sub.G and two .pi.-phase signals E.sub.R and E.sub.B in a ratio of 0.59: 0.41, it is impossible to precisely equalize the signal amount ratio of zero-phase signal E.sub.G to .pi.-phase signal E.sub.R +E.sub.B with 1:1 because of restrictions on the luminance signal of the NTSC system, but a higher resolution can be achieved.
Since a change in the color temperature conditions with the white balance suitably adjusted collapses the relationship expressed by equations (1) and (2), this reduces the high resolution effect. In this case, if the white balance is readjusted under new color temperature conditions, it is possible to return the signal amount ratio of zero-phase signal E.sub.G to .pi.-phase signal E.sub.R +E.sub.B to the original ratio 0.59:0.41.
Under illumination of high color temperatures, the output level of the R-component solid-state imaging element 13 is low because there are few red components from the beginning. Therefore, to adjust the white balance, it is necessary to amplify the output signal of the solid-state imaging element 13 to as high a level as meets equation (1).
However, greatly amplifying the output signal of solid-state imaging element 13 means that the signal-to-noise (SN) ratio of the output signal of solid-state imaging element 13 is degraded, and consequently the SN ratio of the luminance signal is deteriorated.
Even if the white balance is suitably adjusted under the standard illumination, the signal amount ratio of zero-phase signal E.sub.G and .pi.-phase signals E.sub.R +E.sub.B constituting the luminance signal approaches 0.59:0.11 because, for example, a cyanic subject has almost no red component from the beginning. As a result, zero-phase signals E.sub.G and .pi.-phase signal E.sub.R +E.sub.B become unbalanced in signal level, which weakens the offset effect of moire components produced by solid-state imaging elements 14, and 13 and 15 half a pixel pitch horizontally shifted from each other, leading to the disadvantage that the high resolution effect decreases rapidly.
As described above, the conventional three-plate solid-state color imaging apparatus encounters the problem that, when the color temperature conditions for the subject change during shooting, zero-phase signal E.sub.G and .pi.-phase signals E.sub.R +E.sub.B become unbalanced in signal level, resulting in a decrease in the high resolution effect.
At this time, readjusting the white balance under new color temperature conditions allows the signal amount ratio of zero-phase signal E.sub.G and .pi.-phase signal E.sub.R +E.sub.B to return to the original ratio. However, it is necessary to amplify the R-component signal greatly because, for example, the output level of the R-component solid-state imaging element 13 is low from the beginning under illumination of high color temperatures. This degrades the SN ratio of the R component signal, and consequently the SN ratio of the luminance signal.
Further, even if the white balance has been suitably adjusted under the standard illumination, the signal amount ratio of zero-phase signal E.sub.G to .pi.-phase signal E.sub.R +E.sub.B constituting the luminance signal has approached 0.59:0.11 because, for example, a cyanic subject has almost no red component from the beginning. As a result, zero-phase signal E.sub.G and .pi.-phase signal E.sub.R +E.sub.B has become unbalanced in signal level, which has reduced the high resolution effect.